Subsea pipe-in-pipe structures

ABSTRACT

A rigid pipe-in-pipe structure for subsea transportation of fluids includes inner and outer pipes defining a thermally-isolating annulus between them. Thermal insulation material is disposed in the annulus. The outer pipe is of metal, preferably carbon steel. The inner pipe is a polymeric composite structure of bonded layers including a first, radially outer tubular polymeric electrically insulating layer, which can be of pure polymer, surrounding a second, composite layer including carbon fibers, preferably continuous fibers, embedded in a polymer matrix. Conveniently the second layer is a heating layer in which the carbon fibers conduct electricity to heat the inner pipe. The inner pipe can also comprise a radially inner tubular polymer layer within the second composite layer.

This invention relates to rigid pipelines of pipe-in-pipe (‘PIP’)construction that are suitable for subsea applications as used inoffshore oil and gas production.

Subsea pipelines are used as ‘tie-backs’ to transport crude oil and/ornatural gas from a subsea wellhead across the seabed on the way to thesurface. Typically, in offshore locations, the oil and gas flows up ariser from the seabed to the surface to undergo treatment and temporarystorage at a surface installation.

Conventionally, pipelines for transporting oil and gas underwater arefabricated or assembled from steel pipe joints attached end-to-end, forexample by welding. This is the most common example of what is known inthe art as a rigid pipeline. The steel is isolated from seawater by ananti-corrosion coating, which may be of plastics or concrete. A coatingaround the pipe may also be thermally insulating to contribute tothermal management of the fluids flowing within.

Oil and gas are present in subterranean formations at elevatedtemperature and pressure, which may be increased by the injection offluids such as steam. On production of the oil or gas, the producedfluid emerges from the wellhead and enters a subsea pipeline in amulti-phase state.

During subsequent transportation along the pipeline, the temperature andpressure of the produced fluid have to be kept high enough to ensure asufficient flow rate across the seabed and up the riser. In particular,various measures are taken to ensure that the internal temperature ofthe pipeline remains high, typically above 65° C. and in some casesabove 200° C., despite thermal exchange with seawater which, forexample, is around 4° C. below 500 m depth.

Low temperature increases the viscosity of the produced fluid andpromotes precipitation of solid-phase materials, namely waxes andasphaltenes in crude oil and hydrates in natural gas. Such solid-phasematerials tend to deposit on the inner wall of the pipeline and mayeventually cause plugs, which will interrupt production. Aside from thehigh cost of lost production, plugs are difficult and expensive toremove and can even sever the pipeline.

In addition, an oil or gas field must occasionally be shut down formaintenance. During shut-down, production is stopped and so no hot fluidflows through the pipeline. Consequently, to avoid clogging bysolid-phase materials, mitigating fluid such as methanol or diesel oilis injected into the pipeline during shut-down. When productionrestarts, temperature within the pipeline must be increased quickly sothat no plugs will form.

It is important to maintain thermal management continuously along thelength of a pipeline. Otherwise, ‘cold spots’ will arise, which increasethe likelihood of plugs forming at those locations. The challenges ofthermal management increase as subsea pipelines become longer. In thisrespect, there is a trend toward longer tie-backs as oil and gasreserves are being exploited in increasingly challenging locations.

In addition to ensuring effective thermal management, subsea pipelinesmust meet several other technical challenges simultaneously. They mustbe installable by existing installation vessels and thereafter mustremain reliable for many years of service while allowing a sufficientflow rate of produced fluid. They must withstand mechanical loads duringinstallation and afterwards in situ on the seabed, where they mustwithstand hydrostatic pressure exerted by the surrounding seawater andalso the high temperature and pressure of the produced fluid, especiallynear the wellhead. Installed pipelines must also withstand mechanicalloads including those induced by thermal expansion and contraction aswell as by friction with the seabed, while ensuring fluid-tightinterfaces with other subsea equipment. In use, installed pipelines mustalso resist external corrosion from seawater and internal corrosion fromthe produced fluid.

As many of these design constraints are incompatible, trade-offs have tobe made. For example, as water depth increases, hydrostatic pressureincreases by nominally 1 bar for every 10 m depth, while seawatertemperature decreases until it stabilises at around 4° C. below 500 mdepth. Simultaneously, the length and hence the weight of the pipelinecatenary suspended between the installation vessel and the seabedincreases, meaning that thermal management has to be more efficient andthat the installation vessel has to be more capable.

These conflicting challenges have led to the development of variousalternatives to conventional rigid and flexible pipelines, for example:

-   -   for small diameters, reinforced polymer pipelines have been        adopted, in particular fibre-reinforced pipelines and pipelines        of composite material such as in WO 2012/072991, which teaches a        composite pipeline comprising a matrix containing embedded        fibres that may have variable properties along its length;    -   lined pipes, in particular plastics-lined pipes and pipes lined        with a corrosion-resistant alloy, for example as described in EP        0990101;    -   bundled pipelines; and    -   PiP structures comprising a fluid-carrying steel inner pipe        positioned concentrically within a steel outer pipe, for example        as disclosed in U.S. Pat. No. 6,145,547.

In PiP structures, the inner and outer pipes are spaced from each otherto define a thermally-insulating annulus between them. Typically,thermally-insulating material is disposed in the annulus; it is alsopossible to draw down a partial vacuum in the annulus to reducetransmission of heat through the annulus. In these ways, the annulusprovides high-performance thermal insulation in a passive approach tothermal management. The double-walled construction also enhancesmechanical strength and leak protection.

PiP pipelines may be fabricated offshore on, and laid from, a pipelayingvessel using J-lay or S-lay techniques. PiP pipelines may also be laidin reel-lay operations, in which the pipeline is prefabricated in longstalks at a coastal spoolbase that a reel-lay vessel visits periodicallyfor loading. At the spoolbase, the stalks are joined end-to-end as theresulting pipeline is spooled onto a reel carried by the vessel.

Spooling imparts plastic deformation to the pipeline. During subsequentpipelaying at sea, the pipeline is unspooled from the reel andstraightened to recover the plastic deformation. A drawback of steel PiPfor reel-lay applications is that both the inner and outer steel pipesare deformed plastically during spooling, but only the outer pipe can bestraightened properly after unspooling.

Another drawback of steel PiP is its weight: it is so heavy that it isdifficult to install where the water depth exceeds 2000 m. In thisrespect, the outer pipe must bear longitudinal tension loads includingat least some of the weight of the inner pipe and the surroundinginsulation as the pipe string hangs from a hang-off mechanism ortensioner on the installation vessel. It will be clear that the innerpipe cannot be engaged directly with a hang-off mechanism or gripped bya tensioner.

An aligner on the installation vessel may partially support the combinedweight of the inner pipe and the insulation that surrounds the innerpipe within the annulus. However, the effective weight of the inner pipeand the surrounding insulation is very substantial, particularly as itarises from their weight in air, which is higher than their apparentweight would be if they were suspended in water like the outer pipe.This places a high load on the aligner, which requires the structuresupporting the aligner to have great rigidity and load capacity.Unhelpfully, this adds weight at an elevated position on a laying towerof the installation vessel, which negatively impacts the stability ofthe vessel for a given size of hull. Self-weight also exerts asubstantial compressive stress on the inner pipe near the sag bend,where the catenary curves onto the seabed at a touchdown point.

Electrically trace-heated PiP (ETHP) adds active thermal management to aPiP structure to manage the temperature of produced fluids moreeffectively. The trace heating system typically employs resistiveelectrical wires running along, and in thermal contact with, the outersurface of the inner pipe. Heat produced by passing an electric currentalong the wires is conducted through the pipe wall to the produced fluidflowing within. GB 2492883 and WO 2014/029644 disclose typicalelectrically trace-heated PiP flowline sections. However, theseproposals require specific arrangements of heating cables that are noteasy to manufacture and that are therefore expensive. They also sufferfrom the weight of all-steel pipe construction.

Hybrid or composite PiP structures are known in which a steel inner pipetransports a fluid and an outer pipe of composites or plastics definesthe annulus and protects the structure from seawater. This type of pipeis used to transport cryogenic fluids such as liquefied gas.

Other hybrid or composite PiP structures are known in which an innerpipe of composites or plastics transports the fluid and a steel outerpipe defines the annulus and protects the structure from seawater. Forexample, WO 2012/023850 discloses a steel outer pipe and a plasticsinner pipe. This arrangement is limited by the low mechanical strengthof the plastics inner pipe: spacers between the inner and outer pipesare needed to keep the inner pipe centred within the outer pipe and tospread radial loads. Similarly, in U.S. Pat. No. 6,116,290, an innerplastics pipe is ribbed so that the ribs act as spacers. However,spacers may form a thermal bridge between the inner and outer pipes thatwill decrease the thermal performance of the assembly. Additionally, itis difficult to improve the thermal performance of such hybrid PiPstructures by adding active thermal management, because the concentratedlocalised heat of heating elements such as wires could melt a plasticsinner pipe.

EP 2558761 relates to a product offered by Total SA under the trade mark‘Energised Composite Solution’ or ‘ECS’. The PiP structure proposed inEP 2558761 is a composite structure that includes at least one layercomponent made of a composite material. The inner and outer pipes may beof steel or other materials, including composites.

Composites are proposed for the inner and outer pipes in EP 2558761 butsteel has been adopted for those pipes in reality, in the form in whichthe ECS product is currently marketed. FIG. 1 represents the resultingcomposite structure 10 in exploded form, omitting details such aselectrical connections for clarity. The various components of thestructure 10 are rotationally symmetrical around a central longitudinalaxis 12.

In EP 2558761, an inner, fluid-carrying pipe 14 has an outerelectrically-insulating surface. The inner pipe 14 can be of a plasticsmaterial that is inherently electrically insulating on its outersurface, or of steel wrapped with an electrically-insulating layer todefine an electrically-insulating outer surface. In practice, a steelinner pipe 14 wrapped with an electrically-insulating composite layer 16of glass-reinforced polymer is used in embodiments of ECS as currentlymarketed.

A heating layer 18 is wrapped around the inner pipe 14, outside anyelectrically-insulating layer 16 that may also be wrapped around theinner pipe 14. The heating layer 18 is of carbon-reinforced polymercomposite, comprising carbon fibres embedded in a polymer matrix of, forexample, polyamide. A layer of thermal insulation 20 such aspolyurethane foam is placed or injected around the heating layer 18.

A tubular casing in the form of an outer pipe 22 is spaced from theinner pipe 14 by spacers 24 to define an insulating annulus between theinner and outer pipes 14, 22. The spacers 24 are of a reinforcing fillerembedded in a polymer material. In addition to the spacers 24, theannulus contains the heating layer 18, the thermal insulation layer 20and any additional electrically-insulating layers 16. The outer pipe 22has to be strong enough to withstand hydrostatic pressure at greatdepth, for example in excess of 100 bar at depths of greater than 1000m.

In principle, the outer pipe 22 in EP 2558761 is preferably of acomposite material, namely a carbon-reinforced polymer composite.However, steel is mentioned as an alternative material for the outerpipe 22. Again, in practice, a steel outer pipe 22 is used inembodiments of ECS as currently marketed.

For a composite heated PiP structure with a steel outer pipe 22—such asis encompassed by EP 2558761 and marketed as ECS—to work, the outside ofthe heating layer 18 must be electrically insulated from the outer pipe22. Thus, either another electrically-insulating layer 16—for exampleanother composite layer 16 of glass-reinforced polymer—must beinterposed between the heating layer 18 and the thermal insulation 20and spacers 24, or the thermal insulation 20 and spacers 24 mustthemselves be electrically insulating.

It will be evident that practical embodiments of the solution proposedin EP 2558761 are complex: the inner structure 26 comprising the innerpipe 14, the heating layer 18 and the necessary layers of electricalinsulation 16 is a sandwich of non-homogeneous layers that cannot easilybe bonded together. Manufacturing such an inner structure 26 towithstand bending and other stresses of installation and use may bedifficult and expensive. Additionally, the spacers 24 may interfere withthe function of the heating layer 18 and create a direct thermal bridgebetween the heating layer 18 and the outer pipe 22.

Thus, there remains a need for a lightweight, easy to manufacture,substantially rigid PiP structure, preferably with active thermalmanagement that allows heat to be added to the produced fluid.Consequently, the invention proposes a further alternative to rigid andflexible pipelines for deep-water use, namely a PiP structure that ismechanically resistant, chemically resistant, lightweight, easy tomanufacture and to install by conventional means, and cost-effective.

Against this background, the invention resides in a rigid PiP structurefor subsea transportation of fluids. The structure comprises inner andouter pipes in spaced concentric relation to define athermally-isolating annulus between them. Thermal insulation material isdisposed in the annulus such that a gap is left in the annulus betweenthe insulation material and the outer pipe.

In accordance with the invention, the outer pipe is of metal, preferablycarbon steel, suitably being an assembly of at least two steel pipesthat are butt-welded end-to-end. The inner pipe is a polymeric compositestructure of bonded layers, comprising a first, radially outer tubularpolymeric electrically insulating layer, which may be of pure polymer,surrounding a second, composite layer that preferably comprises carbonfibres, more preferably long and continuous carbon fibres, embedded in apolymer matrix. Such a structure is known in the art as a single-wallcomposite pipe structure.

The thickness of the outer layer of the inner pipe varies along itslength such that the inner pipe comprises a series oflongitudinally-spaced formations formed integrally with the outer layerof the inner pipe and protruding radially outwardly from the outerlayer.

Conveniently the second, composite layer is a heating layer. Thus,carbon fibres embedded in the polymer matrix of a composite heatinglayer can conduct electricity to heat the inner pipe and therefore thefluid carried by it. This construction avoids the need to place multipleelectrical wires around the inner pipe, as is done in conventionalheated PiP structures. The carbon fibres of the composite layer aresuitably connected to an electrical power supply at one or more ends ofthe PiP structure.

The first, electrically insulating layer helps to isolate the carbonfibres of the second layer electrically from the metal of the outerpipe. Even when low pressure is applied within the annulus, there willbe no potential discharge as can occur in known heated PiP structures.

Advantageously, the inner pipe also comprises a third bonded layer,namely a radially inner tubular polymer layer within the secondcomposite layer. The third layer is preferably an electricallyinsulating layer, suitably formed of pure polymer. The inner layerconducts heat from the carbon fibres to the fluid but electricallyisolates the carbon fibres from the fluid and has a smooth inner surfaceto assure flow within the PiP structure.

The layers of the inner pipe may be bonded to each other by adhesion,for example via an intermediate adhesive layer, or preferably by fusingof thermoplastic polymers, for example via a welded interface at whichabutting layers melt, soften and/or intermix.

The polymers of the first, second and optional third layers may bethermosets or thermoplastics. For compatibility and structuralintegrity, it is preferred that all of the layers comprise thermosetpolymers or that all of the layers comprise thermoplastic polymers.Thermoplastic polymers are preferred as they enable melt-bonding betweenthe layers. It is further preferred that the polymers of all of thelayers, whether thermosets or thermoplastics, are of the same type. Suchpolymers are advantageously identical or at least structurallycompatible for the purpose of bonding.

References in this specification to polymer or pure polymer allow forthe addition of minor amounts of typical non-polymeric additives orconstituents such as fillers.

The polymers of the first, second and optional third layers mayadditionally serve as a matrix for other reinforcing fibres such asglass fibres, aramid fibres and/or polyolefin base fibres such as afibre sold by DSM Dyneeva B.V. under the registered trade mark‘Dyneema’. Such other fibres are not electrically conductive and neednot be long or continuous; they may, for example, be short, choppedfibres.

The outer diameter of the inner pipe need not be substantially constant.For example, the outer layer of the inner pipe may comprise localoverbuilt protruding formations such as integrally-formedradially-projecting formations protruding from the radially outer sideof the inner pipe. Such formations are suitably made of a materialhaving good thermal and electrical insulation properties and goodmechanical properties. An example is pure or reinforced polymer, whichis preferably of the same type as, or identical to, the polymer orreinforced polymer of the underlying outer layer of the inner pipe.

The protruding formations are spaced longitudinally along the inner pipefor axial location of the insulation material that surrounds the innerpipe between those formations. Those formations also may help tocentralise the inner pipe during its insertion into the outer pipe.Nevertheless, a gap preferably remains between the overbuilt formationsand the interior of the outer pipe, to assist telescopic slidingmovement of the inner pipe within the outer pipe upon assembly and toreduce thermal bridging across the annulus.

Modifying the architecture of the inner pipe in accordance with theinvention by turning it into fully polymeric composite pipe greatlysimplifies manufacture of the PiP structure. It also allows easierintegration of additional functions, for example monitoring by fibreoptics. In this respect, optical fibres can be embedded in or placedbetween any of the two or three abovementioned layers of the inner pipe,such fibres suitably being helically wound around the inner pipe.

As the inner pipe of the invention is lighter and more homogeneousrelative to prior art such as Total SA's Energised Composite Solution(ECS), it can be thicker and stiffer. This means that spacers betweenthe inner and outer pipes are no longer essential, which improvesthermal insulation by avoiding thermal bridging between the inner andouter pipes.

As only the outer pipe of a PiP structure can be straightened to recoverplastic deformation of that steel pipe after unspooling, it isadvantageous if deformation of the inner pipe upon spooling is confinedto the elastic domain. The composite structure of the inner pipe makesthis possible.

The thermal insulation material used in PiP structures of the inventioncan be of any known type but is preferably a microporous or nanoporousmaterial, with an exemplary pore size in the range of 10 to 200 microns,so that a vacuum can be drawn in the annulus to improve thermalperformance. For example, the pressure in the annulus may be reduced tobelow 100 mbar. Examples of suitable thermal insulation materials areaerogels and flexible panels of pyrogenated silica as sold by MicrothermNV under the registered trade mark ‘Izoflex’. The thermal insulationmaterial is suitably also electrically insulating, thus having thebeneficial side-effect of helping to isolate the inner pipeelectrically.

As the composite inner pipe has a weight in air that is drasticallylower than that of an equivalent steel pipe—being about 20% of thedensity of steel—the weight load applied by the inner pipe to an alignerand hence to the vertical or inclined laying tower of an installationvessel is greatly reduced. This allows the aligner and the laying towerto be less massive, thus enabling the vessel to be stable with a smallerhull. Compressive stress of the inner pipe near the sag bend due toself-weight of the inner pipe is also greatly reduced.

The inventive concept also embraces a method for manufacturinglightweight PiP structures for subsea transportation of fluids. Broadly,that method comprises:

-   -   providing an inner pipe, being a polymeric composite structure        of bonded layers comprising a first, radially outer polymeric        layer surrounding a second layer of composite material        comprising reinforcing fibres embedded in a polymer matrix;    -   varying the thickness of the outer layer of the inner pipe along        its length to produce longitudinally-spaced outwardly-protruding        formations;    -   placing the inner pipe into a metal outer pipe to leave a        thermally-isolating annulus between the inner and outer pipes;        and    -   providing thermally-insulating material around the inner pipe,        wherein a gap is left in the annulus between the insulation        material and the outer pipe.

For example, the method may involve fabricating or otherwise making aninner multi-layer composite pipe of at least two bonded layers, namelyan outer electrically insulating polymeric tube disposed around acomposite layer containing at least long or continuous carbon fibresembedded in a polymer matrix; and inserting the inner pipe into a metalouter pipe, which may be assembled by welding together successive steelpipes, to leave a thermally-isolating annulus between the inner andouter pipes.

The method may also involve wrapping at least one layer ofthermally-insulating material around the inner pipe and then insertingthe inner pipe wrapped with the thermally-insulating layer into theouter pipe. Alternatively a thermally-insulating material could beplaced in the annulus after the inner pipe is inserted into the outerpipe, for example by injecting or pouring the material through anopening in the wall of the outer pipe.

A third bonded layer, suitably of pure polymer, may be provided as aninner layer of the inner pipe.

The inner pipe is preferably manufactured in a single section. Forexample, the inner pipe may be manufactured by co-extrusion involvingsimultaneous extrusion of two or more layers.

The local protruding formations may simplify wrapping of thermalinsulation material and insertion of the inner pipe into the outer pipe.Thermal insulation material may be placed between longitudinally-spacedprotruding formations.

After closing ends of the annulus, for example with bulkhead structuresthat join the inner and outer pipes, air may be pumped out of theannulus until pressure in the annulus is sufficiently low to achieve adesired level of thermal insulation.

In the foregoing description, mention has been made of rigid pipes. Itis important to understand that in the lexicon of the subsea oil and gasindustry, nominally ‘rigid’ pipes have enough flexibility to be bentalong their length if a minimum bend radius (MBR) is observed. Yet, suchpipes are not regarded in the industry as being ‘flexible’.

Examples of rigid pipes used in the subsea oil and gas industry arespecified in the American Petroleum Institute (API) Specification 5L andRecommended Practice 1111. A rigid pipe usually consists of or comprisesat least one solid steel pipe, although rigid pipes of compositematerials have recently been proposed. Additional components can beadded to form a composite structure, such as an internal liner layer oran outer coating layer. Such additional components can comprise polymer,metal or composite materials. Rigid pipe joints of steel are typicallyterminated by a bevel or a thread, and are assembled end-to-end bywelding or screwing them together.

The allowable in-service deflection of rigid steel pipe is determined bythe elastic limit of steel, which is around 1% bending strain. Exceedingthis limit causes plastic deformation of the steel. It follows that theMBR of rigid pipe used in the subsea oil and gas industry is typicallyaround 100 to 300 meters depending upon the cross-sectional dimensionsof the pipe. However, slight plastic deformation can be recovered orrectified by mechanical means, such as straightening. Thus, duringreel-lay installation of a rigid pipeline made up of welded rigid pipejoints, the rigid pipeline can be spooled onto a reel with a typicalradius of between 8 and 10 meters. This implies a bending strain around2% for conventional diameters of rigid pipes, requiring the pipeline tobe straightened mechanically upon unspooling.

Alternatively, flexible unbonded pipes used in the subsea oil and gasindustry are specified in API Specification 17J and Recommended Practice17B. The flexible pipe body is composed of a composite sandwich-wallstructure of layered materials, in which each layer has its ownfunction. Typically, polymer tubes and wraps ensure fluid-tightness andthermal insulation. Conversely, steel layers or other elements providemechanical strength to resist tensile loads and hydrostatic pressure;for example, interlocked steel tapes form a carcass or pressure vaultand a tensile armour is formed of helically-wound wire. Flexible pipesare terminated and assembled by end fittings. Unlike rigid pipelinesthat are assembled from multiple pipe joints, flexible pipelines aretypically manufactured continuously to the desired length between theirend fittings.

The structure of a flexible pipe allows a large bending deflectionwithout a similarly large increase in bending stresses. The bendinglimit of the composite structure is determined by the elastic limit ofthe outermost plastics layer of the structure, typically the outersheath, which limit is typically 6% to 7% bending strain. Exceeding thatlimit causes irreversible damage to the structure. Consequently, the MBRof flexible pipe used in the subsea oil and gas industry is typicallybetween 3 and 6 meters.

The foregoing description also mentions both composite materials andcomposite structures. Those terms are not synonymous: there is animportant distinction between them.

Composite materials (often shortened to ‘composites’) are materials madefrom two or more component materials with different but complementaryproperties. The composite nature of a composite material is intrinsic tothat material itself. The component materials remain distinct from eachother in the composite material but, when working together, thecomponent materials confer characteristics on the composite materialthat are different from those of the individual component materials. Anexample of a composite material is a reinforced plastics material suchas a fibre-reinforced polymer, being a combination of reinforcing fibresembedded in a matrix of polymer.

In contrast, composite structures are structures made of two or morecomponents of different materials. The material of each component ischosen to perform the function of that component and to optimise thestructure as a whole. Examples of composite structures used in thesubsea oil and gas industry are a layered flexible pipe or a lined orcoated rigid pipe, each of which typically comprises one or morecomponents of carbon steel and one or more other components of differentmaterials such as polymers or corrosion-resistant alloys. The compositestructure may include one or more components made of compositematerials, but not necessarily so. Thus, the composite nature of acomposite structure is extrinsic to any of the materials from which thecomponents of that structure are made.

Reference has already been made to FIG. 1 of the appended drawings,which is a schematic exploded longitudinal sectional view of anelectrically trace-heated PiP structure known in the prior art. In orderthat the invention may be more readily understood, reference will now bemade, by way of example, to the remaining drawings in which:

FIG. 2 is a schematic exploded longitudinal sectional view of anelectrically trace-heated PiP structure in accordance with theinvention, including a layered bonded inner pipe having an optionalinner layer;

FIG. 3 corresponds to FIG. 2 but in a non-exploded form;

FIG. 4 corresponds to FIG. 3 but shows a variant of the invention inwhich overbuilt formations project radially outwardly from an outerlayer of the inner pipe; and

FIG. 5 is a schematic diagram of a spoolbase performing a method of theinvention to assemble a PiP structure of the invention.

In FIGS. 2 to 4, like numerals are used for like parts. In particular, aheating layer 18 being a composite of continuous carbon fibre in apolymer matrix, a layer of thermal insulation 20 and a metal outer pipe22 are apparent in PiP structures 28 of the invention. The outer pipe 22is of carbon steel and is conveniently assembled from steel pipe jointsthat are butt-welded end-to-end.

In each of FIGS. 2 to 4, the heating layer 18 is incorporated into aninner pipe 30, which is a composite structure of polymeric orpolymer-based layers bonded together to form a solid pipe. The innerpipe 30 further comprises an electrically insulating outer layer 32surrounding and bonded to the heating layer 18.

In this example, the inner pipe 30 also comprises an optionalelectrically-insulating inner layer 34 bonded on the radially inner sideof the heating layer 18. The inner layer 34 presents a smooth innersurface that defines a flow path for the fluids carried by the PiPstructure 28 in use.

The outer layer 32 and the optional inner layer 34 of the inner pipe 30may be of non-reinforced polymer, which may be expressed as a purepolymer, or may comprise a polymer matrix reinforced by the addition offibres such as glass fibres that are not electrically conductive.

In the heating layer 18 and preferably also in the other layers 32, 34if they also contain reinforcement fibres, the reinforcement fibres areindividually embedded in the polymer matrix to create a solid pipe wallthat is impervious to gas even at high pressure. This is unlike a knowncomposite pipe technology called Reinforced Thermoplastic Pipe or RTP,in which dry fibre reinforcements are wound around a liner as rovings.As the individual fibres of the rovings in RTP are not embedded in amatrix and so remain dry, the pipe wall of RTP is not truly solid. Thismeans that gas can accumulate within the pipe wall, which makes RTPunsuitable for carrying fluids at high pressures. In contrast, thethickness, configuration and materials of the inner pipe 30 arepreferably designed to withstand an internal fluid pressure of more than100 bar, more preferably over 200 bar or even possibly over 300 bar, andan internal fluid temperature of more than 150° C.

The inner pipe 30 is advantageously manufactured as a single continuousintegrated component. For example, the inner pipe 30 may be manufacturedby co-extrusion, involving simultaneous extrusion of two or more of thelayers 18, 32, 34. This has the further advantage over RTP that theinner pipe 30 can be made continuously to any desired length, such as5,000 m to 10,000 m, whereas the length of an RTP component is limitedto typically 400 m in view of limitations of its manufacturing process.This is too short for many subsea tie-backs if the inner pipe 30 is tobe in one continuous piece.

The polymer materials of the layers 18, 32, 34 are preferablythermoplastics to allow melt-bonding at their interfaces to form aninner pipe 30 that is solid through its full wall thickness. The polymermaterials of the layers 18, 32, 34 are preferably compatible for thepurpose of bonding to each other and are more preferably identical.Examples of polymers that may be used in the inner pipe 30 arepolyethylene (PE), polypropylene (PP), polyamide (PA), polyvinylidenedifluoride (PVDF) and polyether ether ketone (PEEK).

As spacers 24 shown in FIG. 1 may preferably be omitted in PiPstructures of the invention, the layer of thermal insulation 20 canextend continuously along the annulus between the inner pipe 30 and theouter pipe 22 as shown in the embodiment of FIGS. 2 and 3. Further toimprove thermal management, a gap 36 is preferably left in the annulusbetween the insulation 20 and the outer pipe 22 as seen in FIG. 3.

FIG. 4 shows another PiP structure 28 of the invention. Here, protrudingformations 38 are spaced longitudinally along the outside of the outerlayer 32 of the inner pipe 30. The formations 38 extendcircumferentially around the inner pipe 30 and protrude radiallyoutwardly from the outer layer 32 to near the inside of the outer pipe22. The formations 38 help to centralise the inner pipe 30 during itsinsertion into the outer pipe 22. To minimise friction or jamming duringinsertion and to reduce thermal bridging in use, the formations 38 taperoutwardly to respective narrow apices, for example with the rounded orradiused section shown in FIG. 4. Also, the formations 38 remain spacedat their apices from the inside of the outer pipe 22.

The formations 38 can have any useful shape, for example a trapezoidalshape, an outwardly-tapered shape, a tooth shape or a rounded shape inaxial, longitudinal cross-section.

The formations 38 are of pure or reinforced polymer, which is preferablycompatible with or identical to the polymer of the outer layer 32. Theformations 38 could be formed integrally with the outer layer 32 as theinner pipe 30 is manufactured. For example, an extrusion die forming theouter layer 32 in a co-extrusion process could be expanded andcontracted radially at intervals to create the formations 38 as theinner pipe 30 advances during extrusion. Alternatively, the formations38 could be bonded to or over-moulded on the outer layer 32 after theunderlying layers of the inner pipe 30 are manufactured. For example,bonding may be achieved by fusing or by use of an adhesive.

In this example, the protruding formations 38 interrupt the layer ofthermal insulation 20 in the annulus between the inner pipe 30 and theouter pipe 22. The formations 38 provide axial location for theinsulation 20 to resist longitudinal slippage of the insulation 20relative to the inner pipe 30.

FIG. 4 also shows optical fibres 40 that can be embedded in or placedbetween any of the layers 18, 32, 34 of the inner pipe 30 for monitoringor data-carrying purposes. Optical fibres 40 are suitably woundhelically around the inner pipe 30 and so are seen here incross-section.

FIG. 5 represents a coastal spoolbase 42 performing a method of theinvention to assemble a PiP structure of the invention.

The PiP structure 28 of the invention is apt to be built onshore at thespoolbase 42 or at a yard in stalk assemblies of considerable length,which may for example be longer than 500 m and possibly 1000 m or more.The outer pipe 22 can be fabricated to the desired length by weldingtogether a succession of pipe joints 44, either around the inner pipe 30or, as shown in FIG. 5, as a prelude to telescopic insertion of theinner pipe 30 into the prefabricated outer pipe 22.

The inner pipe 30 can be brought to the spoolbase 42 either as anoversized continuous element that can be cut to a desired length at thespoolbase 42 or as a discrete element of predetermined length to suitthe desired length of the finished PiP structure 28. FIG. 5 shows thefirst option. The inner pipe 30 is conveniently transported to thespoolbase 42 in a compact curved configuration, for example spooled ontoa reel or carousel 46 as shown in FIG. 5.

Overbuilt protruding formations 38 of the required dimensions may beovermoulded or otherwise bonded onto the inner pipe 30 at the spoolbase42, if they are not already part of the inner pipe 30 as manufactured.In this instance, after the inner pipe 30 is unspooled from the reel 46at the spoolbase 42, the inner pipe 30 passes through an overmouldingstation 48. There, protruding formations 38 are overmoulded onto theouter layer 32 of the inner pipe 30 at longitudinally-spaced intervals.

Next, the inner pipe 30 passes through an insulating station 50 at whicha layer of insulating material 20 is wrapped around the inner pipe 30between the longitudinally-spaced protruding formations 38. Thusinsulated, the inner pipe 30 is inserted telescopically into theprefabricated outer pipe 22.

Finally, bulkheads 52 at each end of the PiP structure 28 join therespective ends of the outer and inner pipes 22, 30 to close the annulusbetween those pipes 22, 30. Air can then be pumped out of the annulus todraw down a partial vacuum if required.

The PiP structure 28 can be towed from the spoolbase 42 to an offshoreinstallation location or spooled onto a reel of a pipelay vessel to becarried to an offshore installation location before unspooling andlaying. The PiP structure 28 may be a stalk of a longer reel-laypipeline, in which case it may be welded via the bulkheads 52 to one ormore similar adjoining PiP structures 28 placed end-to-end. Structuressuch as towheads or accessories such as tees or termination structurescan be welded to the end of the PiP structure 28 via a bulkhead 52.

Other variations are possible within the inventive concept. For example,thermally-insulating material could be placed in the annulus after theinner pipe 30 is inserted into the outer pipe 22, for example byinjecting or pouring the material through an opening in the wall of theouter pipe 22.

Optical fibres 40 can be arranged in any alternative configuration knownto the skilled person, such as in straight lines in the longitudinaldirection or in a wave pattern. Also, the structure 28 may comprisediscrete sensors embedded within or between any of the layers 18, 32,34, an example of such a sensor being a Fibre Bragg Grating (FBG)reflector that is sensitive to temperature and/or to strain.

The heating layer 18 could be replaced with a corresponding compositelayer in which the reinforcing fibres are not electrically heated.However, it is convenient and preferred that the reinforcing fibres of aheating layer 18 are used for heating PiP assemblies 28 of theinvention.

The invention claimed is:
 1. A rigid pipe-in-pipe structure for subseatransportation of fluids, comprising: inner and outer pipes in spacedconcentric relation to define a thermally-isolating annulus betweenthem; and thermal insulation material disposed in the annulus, wherein agap is left in the annulus between the insulation material and the outerpipe; wherein the outer pipe is made of metal and the inner pipe is apolymeric composite structure of bonded layers comprising a first,radially outer polymeric layer surrounding a second layer of compositematerial comprising reinforcing fibres embedded in a polymer matrix, andwherein the thickness of the outer layer of the inner pipe varies alongits length such that the inner pipe comprises a series oflongitudinally-spaced formations formed integrally with the outer layerof the inner pipe and protruding radially outwardly from the outerlayer.
 2. The structure of claim 1, wherein the reinforcing fibres ofthe second layer comprise carbon fibres.
 3. The structure of claim 2,wherein the second layer is a heating layer in which the carbon fibresare connected to an electrical power supply for resistant heating. 4.The structure of claim 1, wherein the inner pipe further comprises athird bonded layer being a radially inner polymeric layer within thesecond composite layer.
 5. The structure of claim 1, wherein thepolymers of the layers of the inner pipe are either all thermosetpolymers or all thermoplastic polymers.
 6. The structure of claim 5,wherein the polymers of all of the layers are structurally compatiblepolymers.
 7. The structure of claim 1, wherein the polymer of one ormore layers of the inner pipe serves as a matrix for reinforcing fibresthat are electrically insulating.
 8. The structure of claim 1, whereinabutting layers of the inner pipe are bonded to each other by fusing ofthermoplastic polymers of those layers.
 9. The structure of claim 1,wherein a gap remains between the protruding formations and the Interiorof the outer pipe.
 10. The structure of claim 1, further comprising atleast one optical fibre embedded in or placed between one or more of thelayers of the inner pipe.
 11. The structure of claim 1, wherein theannulus is maintained without spacers between the inner and outer pipes.12. The structure of claim 1, wherein the thermal insulation material ismicroporous or nanoporous.
 13. The structure of claim 1, wherein thethermal insulation material is electrically insulating.
 14. Thestructure of claim 1, wherein the annulus Is evacuated to a pressure ofless than 100 mb.
 15. A subsea installation comprising at least onepipe-in-pipe structure as defined in claim
 1. 16. A method formanufacturing a rigid pipe-in-pipe structure for subsea transportationof fluids, the method comprising: providing an inner pipe, being apolymeric composite structure of bonded layers comprising a first,radially outer polymeric layer surrounding a second layer of compositematerial comprising reinforcing fibres embedded in a polymer matrix;varying the thickness of the outer layer of the inner pipe along itslength to produce longitudinally-spaced outwardly-protruding formations;placing the inner pipe into a metal outer pipe to leave athermally-isolating annulus between the inner and outer pipes; andproviding thermally-insulating material around the inner pipe, wherein agap is left in the annulus between the insulation material and the outerpipe.
 17. The method of claim 16, comprising wrapping at least one layerof thermally insulating material around the inner pipe and then placingthe inner pipe wrapped with the thermally-insulating layer into theouter pipe.
 18. The method of claim 16, comprising placing athermally-insulating material into the annulus after the inner pipe isplaced into the outer pipe.
 19. The method of claim 16, wherein theinner pipe is manufactured by co-extrusion involving simultaneousextrusion of two or more layers.
 20. The method of claim 16, comprisingfabricating the outer pipe to a desired length by welding together asuccession of pipe joints.
 21. The method of claim 16, comprisinginserting the inner pipe telescopically into a prefabricated outer pipe.22. The method of claim 16, comprising: transporting the inner pipe asan oversized element to an assembly location at which the pipe-in-pipestructure is assembled; and at the assembly location, cutting the innerpipe to a length suiting the structure.
 23. The method of claim 16,comprising transporting the inner pipe as a made-to-length element to anassembly location at which the pipe-in-pipe structure is assembled. 24.A subsea installation comprising at least one pipe-in-pipe structure asmade by the method of claim 16.